1. Field of the Invention
The invention relates to serial printers and, more particularly, to controls for such printers.
Serial printers are printers of the type having either a lesser number of printing elements than is necessary to print across an entire line or having a single print element. In the operation of a serial printer, the print element moves over a certain distance along a line and is actuated for effecting printing at a plurality of locations during its travel. Serial printers, in general, either print one complete character at a time for each actuation of the print element, or they may print by composing a character by means of the actuation of a plurality of wires or other print elements printing less than an entire character, so that the characters are compositions of bars, dots, etc.
2. Description of the Prior Art
Most of the serial printers print a line of characters by beginning from a left margin and sequentially printing until the line is complete. The sheet on which the printing is being accomplished is then indexed, and the printing element is then returned to the left margin before the subsequent line is printed on the sheet. This mode of operation is relatively inefficient due to the time that is required to return the print element to the left margin, substantially reducing the speed of the printer. Therefore, the speed of printing by a serial printer can be substantially increased by indexing the sheet when a line of print is completed and then printing the next line backwards instead of returning the print element to the left margin.
Such bidirectional printing in serial printers has been practiced prior hereto. Early embodiments of bidirectional printers require that the print head proceed from a preset left margin electric switch to preset right margin electric switch before reversing and printing backwards from right to left. Later improvements provided the programmer of the data processing equipment associated with the serial printer the ability to specify a maximum line length for each program. With this arrangement, the serial print head had only to proceed from the left margin switch to the maximum right position specified by the program before reversing itself for printing from right to left. For programs requiring only short line lengths, which all start at the left margin, this technique significantly increased printer throughput.
In order to further increase printing speed, it is known in such bidirectional printers to store information relative to the right and left margins of the next line to be printed. At the completion of a line of printing, a comparison is made between the end position of the line being printed and the right and left margins of the next line to be printed. On the basis of this comparison, the print head is directed to proceed to the nearer margin position for printing the next line.